The injection blow molding system, known internationally as IBM, is widely known and used for the production of plastic containers, mainly for the pharmaceutical industry (vials, nasal dispensers, bottles for tablets, etc.) and the cosmetic industry (roll-on containers, jars for creams, etc.), among other applications.
A conventional injection blow molding system is based on a hydraulic machine with vertical presses for opening and closing the molds, a plasticizing screw for processing the thermoplastic material to be molded, i.e., plasticization of the plastic mass and injection of the molten plastic mass into an injection mold for molding a preform, and a transfer device for transferring the preforms from the injection mold to a blow mold in which the preforms are molded to the final shape of the container by injecting air therein.
The equipment implementing the mentioned conventional injection blow molding system comprise two different molds, a split injection mold which can include a plural number of injection cavities arranged in a horizontal row and a split blow mold which can include the same number of blow cavities arranged in another horizontal row, where the injection cavities define an outer surface of the preforms and the blow cavities define an outer surface of the containers. The split injection mold, the split blow mold, and an ejection station are arranged in the form of an equilateral triangle. The equipment further comprises an equilateral triangle-shaped rotary support plate having, on each of its three sides, a horizontal row of the same number of punches, where the punches define an inner surface of the preforms.
The rotary support plate rotates 120 degrees, and in each position the punches of one of the rows are coupled with the cavities of the injection mold, the punches of another one of the rows are coupled with the cavities of the blow mold, and the punches of the other row are in the ejection station. The preforms injected into the injection mold are transferred to the blow mold, and the finished containers are transferred of the blow mold to the ejection station by means of the punches. As many containers as there are cavities in each mold are produced in each work cycle.
One drawback of this widely used system is that all the injection cavities and all the blow cavities must be located in respective rows, and these rows have a maximum length determined by the amplitude of the machine, which defines the maximum number of cavities that are possible in each row, and therefore the maximum productivity of the equipment. In practice, the length of the molds is between 250 mm in small machines and 1400 mm in large machines. This allows housing, in larger machines, a maximum of 24 to 26 cavities in each row for small-sized containers.
Injection blow molding systems in which both the injection mold and the blow mold are arranged in one and the same frame which can be installed in a horizontal injection machine, which is the type of machine most widely used in the plastic transformation sector due to its great versatility, are also known.
Examples of these compact molding systems are published, for example, in patent documents U.S. Pat. No. 3,492,690 by John E. Goldring et al., and EP 2554355 A1 by Fabio Cantoni, in which a row of injection cavities is flanked by two rows of blow cavities, both the injection and blowing steps being performed simultaneously in cooperation with two rows of punches that are movable in an opening and closing direction and in a transfer direction perpendicular to the rows of punches.
Another known example is patent document EP 2678144 B1 belonging to the same inventor as the present application, which includes one or more rows of cavities where the injection cavities and the blow cavities are arranged in an alternating manner in each row, and where both the injection and blowing steps are preformed simultaneously in cooperation with one or more rows of punches that are movable in an axial opening and closing direction and in a transfer direction parallel to the rows of punches.
These compact systems are often used for small-scale container productions, so these molds usually have from 2 to 8 injection cavities per row.
An important technical aspect that must be taken into account in an injection blow molding system is that the injection cavities and the blow cavities do not work at the same working temperature. The injection cavities work at a relatively high temperature (from 70° C. to 130° C.) for the purpose of keeping the thermoplastic material of the preform at a temperature at which it does not solidify and the preform can thereby be blown, whereas the blow cavities work at a low temperature (from 4° C. to 15° C.) so that once the preform has been blown, the thermoplastic material in contact with the surface of the blow cavity solidifies rapidly, the final container thereby being formed.
The fact that the injection cavities and blow cavities are subject to different temperatures does not constitute a problem in conventional vertical closing injection blow molding systems vertical closure given that the injection cavities and blow cavities are located in different frames that are not in contact with one another. The frame with the injection cavities can thereby be heated with a hot fluid, for example, whereas the frame with the blow cavities can be cooled with a cold fluid, for example.
Although the two frames with cavities are not in contact with one another, the significant difference in temperatures between both frames, which are usually made of steel or aluminium, leads to the occurrence of a different thermal expansion in each frame, and the punches which are arranged in three rows forming an equilateral triangle must fit into both the frame with injection cavities and the frame with blow cavities. Given the length of the frames with cavities, which may reach 1400 mm in larger equipment, as mentioned above, the difference in thermal expansion between the frame with injection cavities and the frame with blow cavities can be of a significant magnitude.
This problem is solved in conventional injection blow molding systems by installing the punches with allowance in the rotary support plate, and by doing so, whenever the mold closes on the row of punches, said punches will be freely positioned in a correct alignment, fitting with the cavities as a result of their allowance and in collaboration with centring elements arranged both in the injection cavities and in the blow cavities.
However, in compact molds for injection blow molding on a horizontal injection machine, the punches cannot be positioned with allowance on their support plate because the quality and centring of the preform and the subsequent uniform wall thickness in the resulting container depend on the position and rigidity of said punches. This creates a problem directly proportional to the dimension of the mold since the magnitude of thermal expansion is proportional to the distance between the end cavities. In the event that the compact mold includes multiple rows of injection cavities and blow cavities arranged in an alternating manner, the magnitude of thermal expansion will be proportional to the distance between the diagonally located end cavities.
U.S. Pat. No. 4,376,090 A discloses an injection and blow mold for an injection machine comprising, aligned in a row, a central injection cavity flanked by two blow cavities and two ejection stations at opposite ends of the row, where the central injection cavity and the two blow cavities are formed in respective blocks fixed to a base plate. The blocks are separated from one another and project from the base plate. Nevertheless, the described cavity arrangement does not allow arranging a plurality of injection cavities and a plurality of blow cavities in an alternating manner and aligned in a row, with blow cavities at opposite ends of the row. This document does not mention the possibility of heating the injection cavity by heating means.
U.S. Pat. No. 4,540,543 A discloses an injection and blow mold for an injection machine comprising rows of molding cavities arranged in a base plate, each row of molding cavities including a number n greater than one of injection cavities and a number n+1 of alternating blow cavities, with blow cavities at opposite ends of the row, a plurality of injection nozzles supplying a molten molding material to the injection cavities, rows of punches arranged in a movable plate, each row of punches including a number 2n of punches, the movable plate being provided with an opening and closing movement in a direction parallel to an axial direction relative to the punches and a transfer movement in a direction perpendicular to the axial direction and parallel to the row of punches, a blow device blowing air into preforms in the blow cavities from outlets of air conduits arranged inside the punches, and an ejection device ejecting finished containers from the punches by means of ejector elements.
U.S. Pat. No. 5,067,891 A discloses an injection and blow mold for an injection machine comprising rows of molding cavities arranged in a base plate, each row of molding cavities including a number n greater than one of injection cavities and a number n+1 of alternating blow cavities, with blow cavities at opposite ends of the row, a plurality of injection nozzles supplying a molten molding material to the injection cavities, rows of punches arranged in a movable plate, each row of punches including a number 2n of punches, rows of pairs of complementary half-molds installed in a movable plate, each row of pairs of complementary half-molds including a number n of pairs of complementary injection half-molds and a number n+1 of alternating pairs of complementary blow half-molds, with pairs of complementary blow half-molds at opposite ends of the row, wherein the complementary injection half-molds and the complementary blow half-molds are cooled by cooling elements through the movable plate, and an ejection device ejecting finished containers from the punches by means of ejector elements.